Vol. 59.] TORRIDONIAN, ETC. OF THE ISLE OF RUM. 209 



principal portion of the exposures. Good examples may be examined 

 in the neighbourhood of Priomh-loch Mor and Loch Sgathaig, and 

 by the roadside north of the latter. These rocks are in themselves 

 devoid of any banded structure, although in their mode of association 

 with other types in the complex they conform with the common 

 parallel disposition. Apart from their mode of occurrence, there is 

 nothing in their appearance to distinguish them from the ordinary 

 granitic rocks of the island and of other islands in the Inner 

 Hebrides. Their essential identity with these is borne out by a 

 microscopic examination. There is a strong tendency to delicate 

 micrographic intergrowths of felspar and quartz, imparting to the 

 rocks the microstructure which is implied in the name ' granophyre,' 

 as used by B-osenbusch. This is the most marked characteristic of 

 the acid plutonic rocks of Tertiary age throughout the British area, 

 the rocks of which I have spoken as granites, although I might 

 with equal propriety follow Sir Archibald Geikie in applying to 

 them as a group the name ' granophyre.' A rock of the type that we 

 are considering has acid felspars and quartz as its chief minerals, 

 and consists principally of micropegmatite. A certain portion of 

 the felspar, and more rarely of the quartz, may form more or less 

 distinct crystals, but the micropegmatite encroaches upon the 

 borders, or even affects the interior, of the crystals. The felspars 

 are orthoclase and oligoclase, of which the former builds the more 

 regular and delicate intergrowths with quartz, while the latter more 

 frequently builds distinct crystals. The ferromagnesian element is 

 sometimes hornblende, sometimes biotite, while magnetite and 

 apatite are found as minor accessory minerals. This description 

 applies equally to the purely-acid portions of the gneissic complex, 

 and to the prevalent type of the large granitic tract of Orval 

 and the neighbouring hills. A medium texture prevails, but 

 some of the rocks have rather a coarse-grained pegmatoid appear- 

 ance in the field. These latter occur as strings or veins in the 

 complex, and on Beinn a' Bharr-shaibh are interposed as a border 

 between the gneissic and non-gneissic rocks. 



The pure granophyric type graduates into another, which is still 

 of thoroughly acid composition, but shows peculiarities which point 

 to the intervention of some element of alien origin. Thus, from the 

 little roadside- section near Loch Sgathaig comes a biotite-bearing 

 granophyre [10492] answering to the above description, while close 

 by, and inseparable from it, occurs a rock of precisely the same 

 character, except that, in addition to the flakes of biotite, it con- 

 tains very numerous little crystal-grains of hypersthene [10493]. 

 The occurrence of a mineral so little expected in a granophyre is 

 significant, and the explanation is not far to seek. It is found in 

 the presence, in the immediate neighbourhood, of abundant inclu- 

 sions or xenoliths of a dark close-grained rock, evidently of basic 

 composition ; and other specimens selected at this place demonstrate 

 very clearly the origin of the pyroxene-grains by reaction between 

 the basalt and the acid magma. These specimens have no marked 

 gneissic banding, and they illustrate in a very instructive way 



